The present invention pertains to an infrared optical gas sensor with at least one infrared radiation source and with at least one infrared radiation detector.
Such an infrared optical gas sensor is disclosed, e.g., in DE 197 13 928 C1 and it contains especially two radiation sources and two radiation detectors as well as associated optical concentrators and a beam splitter.
An essential drawback of the infrared optical gas sensors used hitherto is due to the relatively complex optical design and the cost of the optical components that is associated with it, because two wavelengths, namely, a measuring wavelength and a reference wavelength, are usually used to compensate the effect of the reduction of the radiation intensity of the infrared radiation source due to its aging or due to the contamination of optical surfaces in the beam path. The measuring wavelength (measuring radiation) is selected to be such that the corresponding measured signal of an infrared radiation detector displays a characteristic dependence on the concentration of the gas to be measured. On the other hand, the reference wavelength (reference radiation) is selected to be such that the reference signal measured is affected by the gas to be measured as little as possible. The geometric design of the measuring and reference channels is selected to be such that the radiation will possibly take the same optical path in the measuring and absorption sections for both channels.
The radiation is divided between two separate optical paths for the measuring radiation and the reference radiation in a usually encapsulated area not affected by the gas to be measured, especially by means of a beam splitter. The effect of the above-mentioned changes in the radiation intensity, which affect both channels, is to be eliminated by dividing the signal values of the measuring channel and the reference channel, while the change in the measured signal of the narrow spectral band absorption by the measured gas is preserved.
These hitherto usual measuring arrangements and the corresponding measurement methods have the fundamental drawback that a change in the geometry of the beam distribution between the measuring channel and the reference channel leads to a drift or deterioration of the measured signal. In addition, the measuring effect, i.e., the change in the signal quotient due to absorption, is frequently very small, so that it corresponds to a deviation of the signal quotient by 0.5%, e.g., in the case of a methane sensor based on the required accuracy of 1% of the lower explosion limit. However, such deviations of the signal quotient are likewise easily possible if, e.g., the beam splitter in the optical arrangement does not guarantee ideal splitting of the radiation between the active surfaces of the measuring radiation detector and the reference radiation detector, or if a shift in the image of the radiation source in the plane of the detectors is brought about by aging effects in the radiation source. If the beam spot moves over the edge of the active detector surface, the detector signal will change. A slight asymmetry in illumination thus produces an undesired deviation in the signal quotient. Similar deviations may also be caused by thermal deformations of the sensor structure or by dirt or liquid drops in the beam path, by mechanical shocks or other causes. As an end result, radiation components that are weighted unequally in the measuring and reference channels are thus blanked out.
Complicated technical measures are taken to avoid or reduce the above-described effects. For example, the housing and the carrying structure are made of high-quality metallic materials, and fits prepared with precision and true-to-angle mounts as well as adjustment steps are provided in the manufacturing process. Furthermore, structured intensity distributions in the effective detector surface are smoothened by blurred imaging or by a specific scattering of the radiation. The use of only one detector element with one movable filter wheel arranged in the beam path with different interference filters has been known as well.
The object of the present invention is to propose a simple design for an improved infrared optical gas sensor, which delivers more stable measured signals.
According to the invention an infrared optical gas sensor is provided with at least one infrared radiation source and with at least one infrared radiation detector. The infrared radiation detectors used as reference radiation and measuring radiation detectors are formed of thin layers of a partially transparent material, which delivers an electric measured signal that depends on the radiation intensity received. The infrared radiation detectors are stacked one over the other and are arranged with an intermediate narrow-band filter each, which transmits at the measuring wavelength. The infrared radiation detectors have an electrically conductive coating and are contacted on their top sides and undersides. The measuring radiation detector follows the reference radiation detector in the direction of the beam. The reference radiation detector is transparent for at least part of the measuring radiation at the measuring wavelength.
According to an alternative embodiment of the invention an infrared optical gas sensor is provided with at least one infrared radiation source and with at least one infrared radiation detector. The infrared radiation detectors used as reference radiation and measuring radiation detectors are formed of thin layers of a partially transparent material, which delivers an electric measured signal that depends on the radiation intensity received. The infrared radiation detectors are stacked one over the other and are arranged with an intermediate narrow-band filter each, which stop or block at the measuring wavelength. The infrared radiation detectors on the top side and the underside each have an electrically conductive coating and are contacted. The reference radiation detector follows the measuring radiation detector in the direction of the beam. The measuring radiation detector is transparent for at least part of the reference radiation at the reference wavelength.
One essential advantage of the present invention arises from the compact, layered design of a multiple detector arrangement for the measurement of at least two different wavelengths, namely, a reference wavelength and a measuring wavelength. In the simplest case, the gas sensor according to the present invention may be designed such that the measuring gas holder filled with the gas to be measured is an internally reflecting tube, which has a broad-band infrared radiation source at one end face and a layered multiple detector arrangement according to the present invention at the other, opposite end face. An infrared optical gas sensor with stable output signal is thus provided without an additional imaging optical system. The cylindrical reflector, i.e., the measuring gas holder, ensures increased radiation intensity in the central longitudinal axis of the measuring gas holder and thus also in the entry window of the multiple detector arrangement, which is mounted centrally in one end face of the measuring gas holder.
Contrary to the present invention, a double detector usually used hitherto with entry windows arranged next to one another has drawbacks in this regard due to reduced stability over time and geometric stability of the irradiated effective detector surfaces as well as due to the reduced radiation intensity received because of the impossibility of the geometrically identical arrangement of the two detectors in the central longitudinal axis of the sample holder. The multiple reflections on the cylindrical surface of the measuring gas holder lead to a radiation intensity distribution with a maximum in the central longitudinal axis and a steep drop in intensity in the radial direction. The radiation-sensitive detector surfaces are thus located on the flanks of this intensity distribution, so that the measured radiation intensity strongly depends on the position of the infrared radiation detectors and the change in this position.
The measuring and reference radiation detectors are designed as a thin, partially transparent layer. This may be a thin, plane-parallel disk or layer of a pyroelectric material, which has a preferably transparent, electrically conductive coating on the top side and the underside and is contacted. Suitable pyroelectric materials include especially the crystalline substances lithium tantalate (LiTaO3), strontium barium niobate (SBN), triglycine sulfate (TGS), lead zirconium titanate (PZT), and the polyvinylidene fluoride (PVDF) polymer. It is also possible to use semiconductors, which utilize the inner photo effect for the radiation detection, which is possible in the case of HgCdTe, PbS and PbSe. The design of the detector elements and the contacting must permit the passage of the radiation without shadowing. Transparent conductive layers are especially metallic layers with thicknesses in the range of a few nm or transparent layers such as ITO (indium tin oxide).
A narrow-band filter, which further narrows the spectral component of the incident radiation, is arranged between two layers. A narrow-band filter, which lets through the spectral range containing the measuring and reference wavelengths, may be likewise arranged in front of the first radiation-sensitive layer. The transmission ranges of the filters must be selected to be such that they overlap in at least one partial range. The thickness of the first radiation-sensitive layer must be selected to be such that a sufficient component of the radiation will be both absorbed and transmitted. Layer thicknesses of 20 to 30 xcexcm can be obtained by polishing crystalline materials such as lithium tantalate (LiTaO3). Films made of polyvinylidene fluoride (PVDF) of a comparable layer thickness belong to the state of the art. Layer thicknesses beginning from 1 xcexcm have been obtained for triglycine sulfate (TGS).
The absorption of radiation in the transparent layer is enhanced by a narrow-band filter, which follows in the beam path and is preferably designed as an interference filter, reflecting the component of the radiation that it does not transmit. This component will again pass through the first detector layer and can be absorbed. If the structure comprises more than two partial detectors, this also applies to the succeeding partial detectors.
In the case of the polymer layer consisting of polyvinylidene fluoride (PVDF), a natural selective absorption is present in the range of 3.3 xcexcm, which can be further increased by adding hydrocarbon monomers. With this layer as the first layer in the path of the light, hydrocarbons can be measured selectively without a corresponding narrow band filter having to be arranged in front of it. At the same time, this layer filters out the component of the incident spectrum that is absorbed by hydrocarbons in the atmosphere. As a result, the succeeding detector layer receives the radiation component that is extensively independent from changes in the concentrations of hydrocarbons in the path of the light. This results in a special embodiment of the subject of the present invention.
The last detector layer in the detector structure permits a greater freedom of design. In the case of pyroelectric layers, the entry surface may be blackened in order to achieve complete absorption of the residual radiation. The electrodes do not have to be transparent here. A more complete semiconductor detector with pn transition and electrode structures applied to the reverse side may be used as well.
The strong effect of the thermal environment on the signal must be borne in mind in the case of the use of pyroelectric crystals for the individual detector layers. Direct contact of the crystal or film with a narrow-band filter leads to a markedly changed behavior with respect to an unsupported layer especially in the range of low radiation modulation frequencies. The intermediate space must be designed here with respect to the heat conduction from the pyroelectric crystal to the environment such that the sensitivity of the layer becomes optimal. This is achieved by means of suitable substrate materials with low heat capacity and thermal conductivity or by means of an air gap. The thickness of the intermediate space is about 0.3 to 1 mm. The entire layer structure may be accommodated in a usual standard housing, whose entry opening is a diaphragm for the entering radiation. The topmost layer of the structure may be used as an entry window, and this layer may be bonded or soldered to the housing to achieve gas tightness. The diaphragm ensures, among other things, that each entering light beam must travel through all layers of the structure. It is thus ruled out that a component of the radiation reaches only one of the partial detectors and thus leads to a drift of the signal quotient in the case of a change.
To compensate the effect of changing ambient temperatures, one or more additional pyroelectric elements may be added, whose signals are used for negative feedback. To eliminate a sensitivity to the incident radiation, such elements are preferably provided with a gold surface.
For a double detector for measuring CO2, the transmission window for the narrow-band filter in the radiation entry (reference filter) may cover the range of 4.0 to 4.4 xcexcm. The next filter (measuring filter) will then cover the absorption band of CO2 around 4.24 xcexcm with a width of 100 to 200 nm. As an alternative, both transmission ranges may be selected such that they overlap only in the range of the absorption band of CO2, but the reference filter must contain an additional range, in which CO2 does not absorb.
The layer thickness of the infrared radiation detectors according to the present invention is about 1 to 30 xcexcm, and the layer thickness of the narrow-band filters is about 0.3 to 1 mm.
Depending on the particular embodiment of the present invention, the narrow-band filters according to the invention are preferably designed either as interference filters with a narrow transmission range or as interference filters with a narrow stop band or as absorption filters with narrow absorption range. For example, the one narrow-band filter or the plurality of narrow-band filters according to one embodiment is/are preferably an interference filter/interference filters with a narrow transmission range, so that ideally only measuring radiation with the gas-specific wavelength will reach the next layer forming the measuring radiation detector. The other two embodiments of narrow-band filters mentioned are correspondingly suitable preferably for other variants of the present invention.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.